1. Field of the Invention
The present invention relates to optical articles such as lenses, filters and mirrors, and particularly to optical articles having excellent anti-fogging properties and anti-reflection properties.
2. Related Background Art
In order to prevent fogging of lenses, filters, mirrors and the like, it is common to coat their surfaces with a surfactant. Recently, in place of the surfactant, a water-absorbing substance is also applied to an optical substrate to form a film, thereby preventing fogging.
Water-absorbing substances used to prevent fogging include natural and synthetic polymers. The natural polymers include starch-type polymers, such as hydrolysates of starch-acrylonitrile graft polymers, and cellulosic polymers, such as cellulose-acrylonitrile graft polymers, while the synthetic polymers include polyvinyl-alcohol type polymers, such as crosslinked polyvinyl alcohol, acrylic polymers, such as crosslinked sodium polyacrylate, and polyether-type polymers, such as crosslinked polyethylene glycol diacrylate.
However, the anti-fogging optical articles described in the prior art have the following problems.
1. When a surfactant is used to prevent fogging, the lastingness of its effect is so short that the effect cannot be retained unless the surfactant is applied again after several hours or several days. When soil on the surface of the optical article is wiped out with water or the like, the film of the surfactant comes off, and so its effect is lost.
2. When any of the various water-absorbing substances is applied to form a film for the purpose of preventing fogging, the lastingness of its effect is markedly improved compared with the surfactant. However, when the water-absorbing substance is used as the anti-fogging film, and a layer of a substance having a low refractive index is provided thereon for obtaining an anti-reflection effect, the anti-fogging property tends to be impaired.
3. When the thickness of the water-absorbing substance layer is made thin so as to be odd times a fourth of the objective wavelength whose reflection is to be prevented, to serve itself as an anti-reflection film, the film is often too thin to have sufficient anti-fogging properties.
In order to solve the above-described problems, the present inventors found that when a porous film is used as an anti-reflection film, it is possible to provide an anti-reflection film on the water-absorbing film, retaining the anti-fogging effect of the water-absorbing film. However, such a method has many problems: the anti-fogging properties may vary according to the components of the porous film; absorbed moisture and substances in the moisture may remain both in the porous film and at the interface between the water-absorbing film and the porous film; and when the water-absorbing film swells upon water absorption, the porous film may not follow the swelling and may thus be cracked in some cases.
It is an object of the present invention to solve the above-described problems and to provide an optical article that have both anti-fogging and anti-reflection properties, thus to provide an optical instrument in which such an optical article is incorporated to prevent dew condensation.
According to the present invention, there is thus provided an anti-fogging and anti-reflection optical article comprising a substrate, a first water-absorbing film provided on the substrate, a thin film of a high refractive index (herein after xe2x80x9chighly refractive filmxe2x80x9d) provided on the first water-absorbing film, and a second water-absorbing film provided on the highly refractive film, where the first water-absorbing film comprises a water-absorbing polymer provided on the substrate, the highly refractive film comprises a polycondensation product of a hydrolysate of a metal alkoxide, and the second water-absorbing film comprises a second water-absorbing polymer.
The anti-fogging effect of the anti-fogging and anti-reflection optical article according to the present invention is explained as follows.
Moisture in the air is first absorbed by the second water-absorbing film, which is the outermost layer. The moisture absorbed in the second water-absorbing film passes through the highly refractive film (a film of a high refractive index) which contains, as a main component, the polycondensation product of the hydrolyzate of the metal alkoxide, and is absorbed by the first water-absorbing film. When the moisture absorbed in the first water-absorbing film is released to the outside in the reverse route, that is, it passes through the highly refractive film, is absorbed by the second water-absorbing film and is finally released into the air. Such a process can prevent the moisture and substances in the moisture from remaining in the highly refractive film and at the interface between the refractive film and the first or second water-absorbing film. Therefore, failure, such as staining of optical articles caused by the remaining moisture, is prevented.
At the same time, the highly refractive film can also play a role to adhere the first water-absorbing film to the second water-absorbing film. Since this film has higher flexibility compared with general inorganic thin films, it can follow swelling of the water-absorbing films upon water absorption, and so the highly refractive film would not crack by swelling of the water-absorbing films.